In this study, positron emission tomography (PET) is used with a specially developed opioid ligand, 18F-cyclofoxy, to determine the localization and density of mu and kappa opioid receptors in specific brain regions of subjects who are well stabilized methadone maintained former heroin addicts, in order to clarify the neurobiological basis of the addictive diseases, including the role of the endogenous opioid system.